1. Field of the Invention
This invention relates to devices having metallized regions and, in particular, to devices having copper metallized regions.
2. Art Background
A multitude of devices such as multichip modules, printed circuit boards, and hybrid integrated circuits include a patterned region of copper. A clear trend in the manufacture of such devices has been the use of progressively finer patterns, i.e., progressive decrease in metallized line dimension and the spaces between these lines. Presently, typical lines and spaces on printed circuit boards are 150 microns, 50-100 microns on ceramic substrates for multichip modules and at the micron level on silicon devices. The finer the lines and spaces, the greater the density of components and active elements.
The manufacture of devices with copper-containing electrical interconnects such as multichip modules and printed circuit boards is described in compendia such as Thin Film Multichip Modules, G. Messner et al., International Society for Hybrid Microelectronics, Reston, Va. (1992) and Handbook of Printed Circuits, R. Clark, Van Nostrand Reinhold, N.Y. (1985), respectively. For certain substrates such as silicon and ceramics, multilayer metallizations utilizing successive deposition of adhesion-promoting or diffusion barrier metals are required for the overall functioning of the interconnects. Such devices are formed in one approach by depositing a continuous layer or layers of metal, overlying this(ese) metal(s) with a mask such as a patterned polymer mask, and removing by etching the regions of the metal exposed through openings in the mask. A variety of etchants have been employed in this etching procedure. For example, aqueous hydrogen fluoride has been used for the etching of titanium layers, commonly employed as the underlaying bonding layer, while cupric chloride solutions, either containing hydrochloric acid or ammonia, primarily the latter, have been used for etching the thicker copper layers serving as the primary current-carrying component.
Although these solutions have been extensively used, problems still exist in their application as line spacings become smaller. Generally, these etchants rapidly remove material parallel to the substrate at rates comparable to the desired direction normal to the substrate surface. The starting condition is shown in FIG. 1, where 2 denominates the mask material, 3 denominates the metal and 1 denominates the substrate. With the described etchants, the material is removed rapidly towards the substrate and, due to lateral etching, is also etched under the mask material. As a result, configurations such as shown in FIG. 2, are obtained. Clearly, the greater the lateral etching, the larger and less advantageous is the minimum linewidth obtainable. Additionally, as a result of the rapid etch rate, reproducibility is substantially diminished and linewidth control is made more difficult. Thus, in general, it is desirable to produce an etching process which allows greater control with less undercutting.
The problem becomes even more critical when the metal structure to be etched includes a multiplicity of layers of different compositions. For example, metal patterns used in multichip modules to connect components often include an overlying layer of copper, together with an underlying metal layer such as a titanium layer, a palladium-doped titanium layer, or successive palladium and titanium layers. Even if adequate etchants are available for each individual layer, generally, an etchant for one or the other of the metals etches the two at such disparate rates that less than totally desirable results are achieved. Additionally, residues of one or more of the metals often are found in such circumstances. Palladium, interposed between Ti and Cu as metal or Ti-Pd alloy, is not removed by either HF or ammoniacal copper etchants.
For example, in a metallized region on substrate 11, masked by material 14 and having an overlying copper 13 (in FIG. 3) layer and an underlying titanium layer 12, an ammoniacal cupric chloride solution is used to etch the copper, and an aqueous hydrogen fluoride etchant is used to etch the titanium. After the copper is fully cleared using the cupric chloride solution, etching of the titanium with aqueous hydrogen fluoride normally causes substantial undercutting of the titanium layer. Two sources of this undercutting include 1) the immunity of each metal to the other's etchant, thus requiring over-etching to assure complete removal in each step, and 2) formation of a passive oxide left by the copper etchant on the surface of the titanium, requiring an induction period to remove this layer, followed by rapid etch-through and concomitant titanium undercut. As a result, after copper etch, the configuration obtained is shown in FIG. 4, and after the titanium etch, the resulting configuration is shown in FIG. 5. The resulting large undercut in the metal bilayer is certainly not desirable and limits the density of line patterning and utility of the modules.
A few suggestions have been reported for etching metallized regions containing more than one metal layer. For example, as discussed in K. L. James, et al., U.S. Pat. No. 4,345,969, dated Aug. 24, 1982, and M. A. Spak, U.S. Pat. No. 4,220,706, dated Sep. 2, 1980, combinations including strongly oxidizing, concentrated inorganic acids have been employed. Nevertheless, the use of such combinations severely limits substrate and resist composition. It is desirable to have an etchant that allows increased resolution for producing copper lines and that promotes the controlled etching of multilayer metal regions. It would also be environmentally desirable to have an etchant for which recovery of copper and regeneration of used etchant with minimal waste disposal are feasible.